Devices using focused charged particle beams (probe beams) such as scanning electron microscopes (SEM) and focused ion beam (FIB) irradiate a probe beam onto the specimen to make image observations and machine the specimen. Here, the size of the probe cross-section (probe diameter) determines the resolution and machining precision of these charged particle beam apparatuses and basically, the smaller the probe cross section, the higher the resolution and machining precision. Progress has been made in recent years toward developing aberration correctors for focused charged particle beam apparatuses that are now reaching the practical application stage. The aberration corrector utilizes a multi-electrode lens to apply a magnetic field and a non-rotationally symmetrical magnetic field to the beam to give the probe beam an inverse aberration. The aberration corrector in this way cancels out different types of aberrations (or aberrations) such as chromatic aberrations or spherical aberrations generated by the objective lens or deflector lens in the optical system.
Optical systems in devices such as focused charged particle beams of the related art use a rotationally symmetric axial lens. Essentially, the probe diameter can be adjusted to a super-small value by aligning each lens axis and axial aperture, and adjusting the focus and aberration. To adjust the focus and correct the aberration, the probe image was acquired under the condition that the focus was changed and adjustment made by selecting the location with the highest degree of sharpness while comparing the degree of sharpness in the image in at least two dimensions. On the other hand, in devices using focused charged particle beams including aberration correctors, a magnetic field and non-rotationally symmetrical magnetic field were applied by an aberration corrector using a multi-electrode lens. So even though higher-order aberrations do not affect rotationally symmetrical optical systems in the related art, these higher-order aberrations do exert a drastic effect on focused charged particle beam apparatuses. Extracting the maximum possible level of device performance requires accurately measuring the type (aberration component) of aberrations in the beam as well as these higher-order aberrations and the quantity of each aberration component and then removing all aberration components by adjusting the aberration corrector as needed.
Directly observing the cross sectional shape of the probe in focused charged particle beam apparatuses such as SEM and FIB that use probe beams is impossible. A method of the known art therefore extracts information on the probe shape by processing the image in a state where the image from the specimen contains the aberration. The type and quantity of aberration is then found by identifying discrepancies for example in the size, contour, and luminance of a probe shape that contains no aberrations.
In a method for extracting the probe shape as disclosed in JP-T-2003-521801 laid open (patent document 1), specimen structural information is deleted by dividing out the specimen image while underfocused (state where beam converges rearward of specimen) or overfocused (state where beam converges forward of specimen) in the Fourier space from the specimen image while exactly focused (beam is focused on specimen material) to in this way obtain the probe shape. In this method, the probe is gradually made visual while amplifying the aberration component information contained in the probe by shifting the focus. The method in JP-A-2005-183086 laid open (patent document 2) discloses in detail a method for setting the aberration type and quantity from the shape of the probe obtained by the above described technique. The probe shape is in this way found in the underfocus and overfocus states, multiple lines are drawn at equiangular gaps from the median point of the probe shape just obtained, and the line profile information is extracted. Unique quantities expressing the line profile width, bilateral asymmetry, and irregularities (concavities/protrusions) near the center are then found. These unique quantities are next changed by way of the line angles and focus states such as under-focus and over-focus when aberrations are present, and defined as parameters expressing geometrical aberrations and parasitic aberrations (defocus, first-to-third order aberrations, coma, spherical surfaces, frames, stars) up to third-order aberrations where the unique quantities are set as variables for and these quantities are utilized as guides for expressing different aberrations.
In transmission electron microscopes (TEM) on the other hand, an image called a diffractogram is obtained by making Fourier transforms of the amorphous specimen image using an electron beam whose input angle was tilted away from the objective lens optical axis. The diffractogram shape reveals effects from the aberration and so has long been used in attempts to find the aberration coefficient by utilizing the diffractogram image interaction.
A method for example in Ultramicroscopy 81 (2000), pp. 149-161 is disclosed that finds the aberration coefficient by measuring the size of the defocus and aberration from the sloping beam diffractogram and solving the inverse problem. A method is disclosed in Optik 99 (1995) pp. 155-166 for finding the aberration coefficient by calculating the amount of image movement via the beam tilt by calculating the interaction of two images captured under different beam tilt conditions, and solving the inverse problem.